The present invention relates to a polymerization process for preparing syndiotactic polystyrenes without forming agglomeration of the polymers on the inner wall or top wall of the reactor. More particularly, the present invention relates to a polymerization process for preparing syndiotactic polystyrenes in the form of powder of fine particles, which comprises polymerizing styrenic monomers in a batch reactor or in a continuous reactor, recycling a portion of the polystyrene in the reactor and reacting the recycled polystyrene with styrenic monomers, which does not cause agglomeration of the polystyrene on the inner wall or top wall of the reactor.
In general, polystyrenes are classified into an atactic, an isotactic and a syndiotactic structure depending on the position of benzene rings as side chains. An atactic polystyrene has an irregular arrangement of the benzene rings and an isotactic polystyrene has an arrangement that the benzene rings are positioned at one side of the polymer main chain. On the other hand, a syndiotactic polystyrene has a regularly alternating arrangement of the benzene rings.
Metallocene catalysts are used for preparing the syndiotactic polystyrenes. The metallocene catalysts have a bridged structure of a complex of Group IV transition metals such as Ti, Zr, Hf etc. in the Periodic Table and a ligand having one or two cycloalkane dienyl groups such as cyclopentadienyl group, indenyl groups, fluorenyl group, and derivatives thereof. As the metallocene catalysts have high activities, the catalysts can prepare polymers having better physical properties than the Ziegler-Natta catalysts.
A metallocene catalyst is used with a cocatalyst for preparation of the syndiotactic polystyrene. A representative example of the cocatalyst is alkyl aluminoxane which is prepared by reacting water with an alkyl aluminum compound. Such catalyst system can prepare polystyrene having a high syndiotactic stereoregularity and a high molecular weight.
European Patent Publication No. 210 615 A2 (1987) discloses a syndiotactic polystyrene with a good stereoregularity which is prepared by using a catalyst of cyclopentadienyl titanium trichloride or alkylated cyclopentadienyl titanium such as pentamethyl cyclopentadienyl titanium trichloride. Such catalysts are known to have preferable catalyst activity, molecular weight, and syndiotactic index.
Japanese Patent Publication Nos. 63-191811 and 3-250007 disclose sulfur bridged metallocene catalysts which have a low yield of manufacture. Also, Japanese Patent Publication Nos. 3-258812, 4-275313 and 5-105712 disclose alkyl bridged metallocene catalysts which have too low yield of manufacture to commercialize.
U.S. Pat. No. 4,544,762 teaches a process for polymerizing alpha-olefins or styrenes with a high activity and a high stereoregularity using a catalyst system consisting of a transition metal catalyst and a reaction product of alkyl aluminum and metal hydroxide. The catalyst system can prepare polyolefins or polystyrenes with a higher activity and a higher stereoregularity than the Ziegler-Natta catalyst.
Japanese Patent Publication Nos. 62-104818 and 62-187708 disclose metallocene catalysts for preparing polystyrene having a syndiotactic structure. The metallocene catalysts have a transition metal of Group IVB of the Periodic Table and a cyclopentadienyl derivative as ligand. With the catalysts, alkyl aluminoxane is used as cocatalyst, which is a reaction product of an alkyl aluminum with a metal hydroxide.
U.S. Pat. No. 5,026,798 teaches a catalytic process using a Group IVB transition metal component and an aluminoxane component to polymerize alpha-olefins to produce high crystallinity and high molecular weight poly-alpha-olefins.
U.S. patent Ser. Nos. 08/844109 and 08/844110 disclose a new alkyl-bridged binuclear metallocene catalyst, a silyl-bridged binuclear metallocene catalyst, and an alkyl-silyl-bridged binuclear metallocene catalyst to polymerize styrenes to produce polystyrene having high stereoregularity, high melting point, and good molecular weight distribution.
A batch process or a continuation process is adopted to prepare polystyrene having a syndiotactic structure. The processes employ a tank-type reactor equipped with agitating blades. U.S. Pat. No. 5,037,907 discloses a vertical tank-type reactor with an agitating blade. The process is called as solution polymerization because the monomers remain in a liquid phase during polymerization. However, in theoretical, although this process can prevent the polymer from agglomerating because the liquid media are dispersed on the surface of the polymer particles, this type reactor still causes a problem of a low monomer conversion rate such as 75% below which will result in deteriorating quality of the polymer.
U.S. Pat. No. 5,254,647 discloses a self-cleaning reactor which is capable of continuation process and is a twin screw reactive extruder type. The reactor can prevent agglomeration of polymer through mixing. The wiped surface reactor controls polymerization until about 10 to 20% of the polymer product is obtained in a powder state, and a continuation process is carried out in the powder bed reactor which is a vertical tank type. This reactor can prevent a rapid polymerization. However, it is not economical due to use of two reactors and it has a disadvantage for the wiped surface reactor to limit the process capacity of the system.
U.S. Pat. No. 5,484,862 discloses a liquid phase, powder bed polymerization process for preparing syndiotactic polymers from vinyl aromatic monomers comprising continuously introducing one or more vinyl aromatic monomers and one or more catalyst systems to a horizontally disposed, continuously agitated, cylindrically shaped reactor containing solid particles, and continuously removing polymerized product therefrom.
PCT Publication No. 99/10394 discloses a process to produce syndiotactic polymer, which comprises preparing a polymer-containing mixture by polymerizing a first aromatic vinyl monomer with a catalyst at the conversion rate of from 60 to 85% in a first reverse mixing reactor under polymerization condition, and introducing the polymer-containing mixture to a second or more reverse mixing reactors to contact a second aromatic vinyl monomer under polymerization condition.
However, when the conversion rate to polymer is 20% or more, the conventional processes produce large particles having a diameter of 5 mm or more, agglomeration bands by the unflowable products on the top of the reactor, and agglomeration bands at the clearance between the agitating blades and the inner wall of the reactor. The large particles and the agglomeration bands prevent from proceeding with polymerization due to containing monomers and catalysts therein, thereby lowering the production yield and the drying efficiency. Further, the agglomerated material cannot easily be removed because of strong adhesion and causes bad agitation during repolymerization without removing.
Accordingly, the present inventors have developed a process a polymerization process for preparing syndiotactic polystyrenes in the form of powder of fine particles, which comprises polymerizing styrenic monomers in a batch reactor or in a continuous reactor, recycling a portion of the polystyrene in the reactor and reacting the recycled polystyrene with styrenic monomers, which does not cause agglomeration of the polystyrene on the inner wall or top wall of the reactor. In this case of preparing syndiotactic styrenic polymers by adopting the product-recycling process according to the present invention, agglomeration on the inner wall which is one of disadvantages in the bulk polymerization process can be prevented, and styrenic polymers of fine powder with a high bulk density and a uniform particle size can be obtained.
A feature of the present invention is the provision of a process of preparing syndiotactic styrenic polymers with a high conversion rate in the form of fine powder in a reactor which is designed to proceed a powder polymerization gradually by agitating the solid and/or powder reactants in a certain direction and which is a self-cleaning type or a cylindrical type preventing agglomeration of the reactants and/or products on the inner wall or the agitating blades.
Another feature of the present invention is the provision of a process of preparing syndiotactic styrenic polymers in the form of fine powder, which comprises polymerizing styrenic monomers in a reactor, recycling a portion of the polystyrene in the reactor and reacting the recycled polystyrene with styrenic monomers, thereby improving the seed effect.
A further feature of the present invention is the provision of a process of preparing syndiotactic styrenic polymers with a high bulk density in the form of fine powder.
A further feature of the present invention is the provision of a process of preparing syndiotactic styrenic polymers with a high bulk density in the form of fine powder, the process being stable and lower energy consuming.
The above and other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.
The present invention relates to a process of preparing syndiotactic styrenic polymers with a high conversion rate in the form of fine powder, which comprises (a) preparing styrenic polymers in a solid state by reacting a mixture consisting of styrenic monomers, a metallocene catalyst, a cocatalyst and inert organic solvent in a polymerization reactor, (b) separating a portion of the styrenic polymers from the reactor, (c) recycling the portion of the styrenic polymers in the reactor, and (d) reacting the recycled styrenic polymers with styrenic monomers.
The styrenic monomers may include olefinic monomers. The monomers can be introduced to a single inlet or multiple inlets of the reactor. A single reactor or a plural number of reactors can be operated in the present invention. The plural numbers of reactors are arranged in series or in parallel. In the present invention, a self-cleaning mono- or twin-axis reactor can be employed to prevent the polymers from agglomerating on the inner wall or the axis. Alternatively, a cylindrical reactor can be employed, where the styrenic polymers are transported in a solid or powder state rotating in a certain direction at the range of Froude Number of from 0.1 to 10 and polymerization proceeds toward the exit of the reactor.